Advances in plasma processing have facilitated growth in the semiconductor industry. Usually, a plurality of semiconductor devices may be created from dies cut from a single processed wafer (i.e., substrate). Since most recipes for processing the wafer assume that the wafer is planar, a non-planar wafer (e.g., a wafer with a bow) may cause variations that may result in defective semiconductor devices being created.
In an ideal situation, a wafer is perfectly planar. However, most wafers tend to have a slight bow and/or bump, thereby causing the wafer to be non-planar. The non-planarity of the wafer may be due to the original shape of the wafer and/or may be a result of the stress (e.g., mechanical stress) to the film that may have been deposited onto the wafer during one or more deposition steps. In some instances, if the wafer is too non-planar, the wafer may be considered unusable and may be discarded.
During certain processing steps, such as etching, knowing the configuration of the wafer may be important to accurately determine the amount of etching and to prevent the electrode within the processing chamber from accidentally touching the wafer, thereby causing damage to the wafer and/or damage to the electrode. This is especially true for processing chamber that may be sensitive to wafer bow. In an example, a bevel etcher may be especially sensitive to wafer bow since the upper electrode may come within very close proximity to the wafer in order to etch along the edge (e.g., bevel) of the wafer.
In a bevel etcher, the gap between an upper electrode and a wafer may be about 0.35 millimeters. However, the wafer bow may be as big as 0.25 millimeters. Thus, if the wafer bow is not correctly identified, the upper electrode may accidentally touch the wafer, thereby causing damages to the wafer and/or the upper electrode. In addition, since the amount of plasma that man, be introduced into the process module may also depend on knowing the actual gap, not being able to accurately identify the gap may cause variation in the processing.
Hence, before etching may be performed on the wafer, measurements may have to be performed to determine the extent of the wafer bow. However, in-line measurements are generally not taken during the deposition process. Thus, measurement data may not be available to be fed into the etching process in order to determine the extent of the wafer bow. Instead, stand-alone metrology, tools may be employed in order to determine the measurement of a wafer bow. However, the stand-alone metrology tools are usually employed for performing characterization measurement. In other words, each wafer is not measured to determine the wafer bow for the wafer. Instead, a sample may be taken to determine the type of wafer bow that may characterize a cluster of wafers. In addition, since the stand-alone metrology tools are not in-situ or even in-line, the measurements data are usually not in a format that enables the data to be easily fed forward to another tool, such as a bevel etcher.
A method that has been employed to enable in-situ measurements is to include a metrology tool within a process module in order to measure the wafer bow. In an example, measurements of the wafer bow may be taken while the wafer is sitting on an electrostatic chuck within a processing module, while waiting for the etching process to begin. One method of performing this measurement includes shining a beam of light across the wafer and measuring the level of light brightness as the upper electrode of the processing module is lowered to reduce the gap between the upper electrode and the wafer. The lowering of the upper electrode is stopped when a predetermined amount of light is no longer detected. At this point, the upper electrode is determined to be within close proximity to the wafer but not yet touching the wafer.
The purpose of identifying the point at which the upper electrode may be closed to touching the wafer is to determine the minimal distance between the electrode and the wafer, thereby identifying the height of the wafer. Unfortunately, the measurements that are taken are local to a single point. Thus, the measurements may not be the actual height of the water.
Consider the situation wherein, for example, the gap between an upper electrode and a wafer is 0.35 millimeters (as is common in a bevel etcher). Since processing may have to be performed within 0.1 millimeter of the wafer, being able to correctly identify the true height of the wafer may prevent the wafer and/or upper electrode from being damaged. In an example, if the true height of the wafer is 0.25 millimeters; however, the local measurements indicate that the height of the wafer is 0.20 millimeters. Since the processing steps require 0.1 millimeter (in this example) to perform the processing, but the upper electrode is actually 0.05 millimeters from the wafer, the wafer may inadvertently cause the wafer to be etched too deeply, which may cause a bad semiconductor devices to be created.
To facilitate discussion. FIGS. 1A, 1B, and 1C show examples of configurations of a non-planar wafer. FIG. 1A shows a wafer 100 with a bowl shape in which an edge 102 may have a higher height than a center 104. FIG. 1B shows a wafer 110 with a dome shape in which an edge 112 has a lower height than a center 114. FIG. 1C shows a wafer 120 with a wavy shape, like a potato chip, for example, in which the height at each point on wafer 120 may vary.
As can be seen from FIGS. 1A, 1B, and 1C, different configurations may exist for a wafer. Thus, taking measurements at a single point may not be sufficient to determine the true minimal distance between the electrode and the wafer. Even if more than a single point of measurement is taken, the tendency is to measure toward the center of the wafer. However, as can be seen from the figures, the true height may vary depending upon the configuration of the wafer. However, since the measurements are being performed as part of the processing steps, there is usually not sufficient time to take a sufficient sample to determine the true height of the wafer without negatively impacting the total processing time.
In addition, the prior art method does not provide for identifying the thickness of the wafer. In other words, some wafer may be of substandard quality and therefore may be thinner than the traditional thickness of about 0.77 millimeters. Since, the thickness can not be determined with a single point of measurement, the true gap between the upper electrode and the wafer may not be accurately determined. As a result, variation may occur in processing that may result in a defective product.